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          <!---->
          2020
        </a></span></div></div> <div data-v-3ba18f14><main class="page"><div class="page-title" style="display:none;"><h1 class="title"></h1> <div data-v-5d8dbdb4><i class="iconfont reco-account" data-v-5d8dbdb4><span data-v-5d8dbdb4>海星</span></i> <!----> <!----> <!----></div></div> <div class="theme-reco-content content__default" style="display:none;"><blockquote><p>模板方法模式——看看 JDK 和 Spring 是如何优雅复用代码的</p> <p>文章收录在 GitHub <a href="https://github.com/Jstarfish/JavaKeeper" target="_blank" rel="noopener noreferrer">JavaKeeper<svg xmlns="http://www.w3.org/2000/svg" aria-hidden="true" x="0px" y="0px" viewBox="0 0 100 100" width="15" height="15" class="icon outbound"><path fill="currentColor" d="M18.8,85.1h56l0,0c2.2,0,4-1.8,4-4v-32h-8v28h-48v-48h28v-8h-32l0,0c-2.2,0-4,1.8-4,4v56C14.8,83.3,16.6,85.1,18.8,85.1z"></path> <polygon fill="currentColor" points="45.7,48.7 51.3,54.3 77.2,28.5 77.2,37.2 85.2,37.2 85.2,14.9 62.8,14.9 62.8,22.9 71.5,22.9"></polygon></svg></a> ，N线互联网开发必备技能兵器谱</p></blockquote> <h2 id="前言"><a href="#前言" class="header-anchor">#</a> 前言</h2> <p>模板，顾名思义，它是一个固定化、标准化的东西。</p> <p><strong>模板方法模式</strong>是一种行为设计模式， 它在超类中定义了一个算法的框架， 允许子类在不修改结构的情况下重写算法的特定步骤。</p> <h2 id="场景问题"><a href="#场景问题" class="header-anchor">#</a> 场景问题</h2> <p>程序员不愿多扯，上来先干两行代码</p> <p>网上模板方法的场景示例特别多，个人感觉还是《Head First 设计模式》中的例子比较好。</p> <p>假设我们是一家饮品店的师傅，起码需要以下两个手艺</p> <p><img src="https://tva1.sinaimg.cn/large/007S8ZIlly1gigzsl1cnpj32gj0sk4ck.jpg" alt=""></p> <p>真简单哈，这么看，步骤大同小异，我的第一反应就是写个业务接口，不同的饮品实现其中的方法就行，像这样</p> <p><img src="https://tva1.sinaimg.cn/large/007S8ZIlly1gih087fykdj31pk0u0nbj.jpg" alt=""></p> <p>画完类图，猛地发现，第一步和第三步没什么差别，而且做饮品是个流程式的工作，我希望使用时，直接调用一个方法，就去执行对应的制作步骤。</p> <p>灵机一动，不用接口了，用一个<strong>抽象父类</strong>，把步骤方法放在一个大的流程方法 <code>makingDrinks()</code> 中，且第一步和第三步，完全一样，没必要在子类实现，改进如下</p> <p><img src="https://tva1.sinaimg.cn/large/007S8ZIlly1giidk5vt5cj31pf0u0nal.jpg" alt=""></p> <p>再看下我们的设计，感觉还不错，现在用同一个 <code>makingDrinks()</code> 方法来处理咖啡和茶的制作，而且我们不希望子类覆盖这个方法，所以可以申明为 final，不同的制作步骤，我们希望子类来提供，必须在父类申明为抽象方法，而第一步和第三步我们不希望子类重写，所以我们声明为非抽象方法</p> <div class="language-java extra-class"><pre class="language-java"><code><span class="token keyword">public</span> <span class="token keyword">abstract</span> <span class="token keyword">class</span> <span class="token class-name">Drinks</span> <span class="token punctuation">{</span>

    <span class="token keyword">void</span> <span class="token function">boilWater</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
        <span class="token class-name">System</span><span class="token punctuation">.</span>out<span class="token punctuation">.</span><span class="token function">println</span><span class="token punctuation">(</span><span class="token string">&quot;将水煮沸&quot;</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    <span class="token punctuation">}</span>

    <span class="token keyword">abstract</span> <span class="token keyword">void</span> <span class="token function">brew</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>

    <span class="token keyword">void</span> <span class="token function">pourInCup</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
        <span class="token class-name">System</span><span class="token punctuation">.</span>out<span class="token punctuation">.</span><span class="token function">println</span><span class="token punctuation">(</span><span class="token string">&quot;倒入杯子&quot;</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    <span class="token punctuation">}</span>

    <span class="token keyword">abstract</span> <span class="token keyword">void</span> <span class="token function">addCondiments</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    
    <span class="token keyword">public</span> <span class="token keyword">final</span> <span class="token keyword">void</span> <span class="token function">makingDrinks</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
        <span class="token comment">//热水</span>
        <span class="token function">boilWater</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
        <span class="token comment">//冲泡</span>
        <span class="token function">brew</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
        <span class="token comment">//倒进杯子</span>
        <span class="token function">pourInCup</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
        <span class="token comment">//加料</span>
        <span class="token function">addCondiments</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    <span class="token punctuation">}</span>
<span class="token punctuation">}</span>
</code></pre></div><p>接着，我们分别处理咖啡和茶，这两个类只需要<strong>继承</strong>父类，重写其中的抽象方法即可（实现各自的冲泡和添加调料）</p> <div class="language-java extra-class"><pre class="language-java"><code><span class="token keyword">public</span> <span class="token keyword">class</span> <span class="token class-name">Tea</span> <span class="token keyword">extends</span> <span class="token class-name">Drinks</span> <span class="token punctuation">{</span>
    <span class="token annotation punctuation">@Override</span>
    <span class="token keyword">void</span> <span class="token function">brew</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
        <span class="token class-name">System</span><span class="token punctuation">.</span>out<span class="token punctuation">.</span><span class="token function">println</span><span class="token punctuation">(</span><span class="token string">&quot;冲茶叶&quot;</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    <span class="token punctuation">}</span>
    <span class="token annotation punctuation">@Override</span>
    <span class="token keyword">void</span> <span class="token function">addCondiments</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
        <span class="token class-name">System</span><span class="token punctuation">.</span>out<span class="token punctuation">.</span><span class="token function">println</span><span class="token punctuation">(</span><span class="token string">&quot;加柠檬片&quot;</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    <span class="token punctuation">}</span>
<span class="token punctuation">}</span>
</code></pre></div><div class="language-java extra-class"><pre class="language-java"><code><span class="token keyword">public</span> <span class="token keyword">class</span> <span class="token class-name">Coffee</span> <span class="token keyword">extends</span> <span class="token class-name">Drinks</span> <span class="token punctuation">{</span>
    <span class="token annotation punctuation">@Override</span>
    <span class="token keyword">void</span> <span class="token function">brew</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
        <span class="token class-name">System</span><span class="token punctuation">.</span>out<span class="token punctuation">.</span><span class="token function">println</span><span class="token punctuation">(</span><span class="token string">&quot;冲咖啡粉&quot;</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    <span class="token punctuation">}</span>

    <span class="token annotation punctuation">@Override</span>
    <span class="token keyword">void</span> <span class="token function">addCondiments</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
        <span class="token class-name">System</span><span class="token punctuation">.</span>out<span class="token punctuation">.</span><span class="token function">println</span><span class="token punctuation">(</span><span class="token string">&quot;加奶加糖&quot;</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    <span class="token punctuation">}</span>
<span class="token punctuation">}</span>
</code></pre></div><p>现在可以上岗了，试着制作下咖啡和茶吧</p> <div class="language-java extra-class"><pre class="language-java"><code><span class="token keyword">public</span> <span class="token keyword">static</span> <span class="token keyword">void</span> <span class="token function">main</span><span class="token punctuation">(</span><span class="token class-name">String</span><span class="token punctuation">[</span><span class="token punctuation">]</span> args<span class="token punctuation">)</span> <span class="token punctuation">{</span>
    <span class="token class-name">Drinks</span> coffee <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">Coffee</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    coffee<span class="token punctuation">.</span><span class="token function">makingDrinks</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    <span class="token class-name">System</span><span class="token punctuation">.</span>out<span class="token punctuation">.</span><span class="token function">println</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    <span class="token class-name">Drinks</span> tea <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">Tea</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    tea<span class="token punctuation">.</span><span class="token function">makingDrinks</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
<span class="token punctuation">}</span>
</code></pre></div><p>好嘞，又学会一个设计模式，这就是<strong>模板方法模式</strong>，我们的 <code>makingDrinks()</code> 就是模板方法。我们可以看到相同的步骤 <code>boilWater()</code> 和 <code>pourInCup()</code> 只在父类中进行即可，不同的步骤放在子类实现。</p> <h2 id="认识模板方法"><a href="#认识模板方法" class="header-anchor">#</a> 认识模板方法</h2> <p>在阎宏博士的《JAVA与模式》一书中开头是这样描述模板方法（Template Method）模式的：</p> <blockquote><p>模板方法模式是类的行为模式。准备一个抽象类，将部分逻辑以具体方法以及具体构造函数的形式实现，然后声明一些抽象方法来迫使子类实现剩余的逻辑。不同的子类可以以不同的方式实现这些抽象方法，从而对剩余的逻辑有不同的实现。这就是模板方法模式的用意。</p></blockquote> <p>写代码的一个很重要的思考点就是“<strong>变与不变</strong>”，程序中哪些功能是可变的，哪些功能是不变的，我们可以把不变的部分抽象出来，进行公共的实现，把变化的部分分离出来，用接口来封装隔离，或用抽象类约束子类行为。模板方法就很好的体现了这一点。</p> <p>模板方法定义了一个算法的步骤，并允许子类为一个或多个步骤提供实现。</p> <p>模板方法模式是所有模式中最为常见的几个模式之一，是<strong>基于继承</strong>的代码复用的基本技术，我们再看下类图</p> <p><img src="https://tva1.sinaimg.cn/large/007S8ZIlly1gih2dmwutxj31ug0u0h0i.jpg" alt=""></p> <p>模板方法模式就是用来创建一个算法的模板，这个模板就是方法，该方法将算法定义成一组步骤，其中的任意步骤都可能是抽象的，由子类负责实现。这样可以<strong>确保算法的结构保持不变，同时由子类提供部分实现</strong>。</p> <p>再回顾下我们制作咖啡和茶的例子，有些顾客要不希望咖啡加糖或者不希望茶里加柠檬，我们要改造下模板方法，在加相应的调料之前，问下顾客</p> <div class="language-java extra-class"><pre class="language-java"><code><span class="token keyword">public</span> <span class="token keyword">abstract</span> <span class="token keyword">class</span> <span class="token class-name">Drinks</span> <span class="token punctuation">{</span>

    <span class="token keyword">void</span> <span class="token function">boilWater</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
        <span class="token class-name">System</span><span class="token punctuation">.</span>out<span class="token punctuation">.</span><span class="token function">println</span><span class="token punctuation">(</span><span class="token string">&quot;将水煮沸&quot;</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    <span class="token punctuation">}</span>

    <span class="token keyword">abstract</span> <span class="token keyword">void</span> <span class="token function">brew</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>

    <span class="token keyword">void</span> <span class="token function">pourInCup</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
        <span class="token class-name">System</span><span class="token punctuation">.</span>out<span class="token punctuation">.</span><span class="token function">println</span><span class="token punctuation">(</span><span class="token string">&quot;倒入杯子&quot;</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    <span class="token punctuation">}</span>

    <span class="token keyword">abstract</span> <span class="token keyword">void</span> <span class="token function">addCondiments</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>

    <span class="token keyword">public</span> <span class="token keyword">final</span> <span class="token keyword">void</span> <span class="token function">makingDrinks</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
        <span class="token function">boilWater</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
        <span class="token function">brew</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
        <span class="token function">pourInCup</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>

        <span class="token comment">//如果顾客需要，才加料</span>
        <span class="token keyword">if</span> <span class="token punctuation">(</span><span class="token function">customerLike</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
            <span class="token function">addCondiments</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
        <span class="token punctuation">}</span>
    <span class="token punctuation">}</span>

    <span class="token comment">//定义一个空的缺省方法，只返回 true</span>
    <span class="token keyword">boolean</span> <span class="token function">customerLike</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
        <span class="token keyword">return</span> <span class="token boolean">true</span><span class="token punctuation">;</span>
    <span class="token punctuation">}</span>
<span class="token punctuation">}</span>
</code></pre></div><p>如上，我们加了一个逻辑判断，逻辑判断的方法时一个只返回 true 的方法，这个方法我们叫做 <strong>钩子方法</strong>。</p> <blockquote><p>钩子：在模板方法的父类中，我们可以定义一个方法，它默认不做任何事，子类可以视情况要不要覆盖它，该方法称为“钩子”。</p></blockquote> <p>钩子方法一般是空的或者有默认实现。钩子的存在，可以让子类有能力对算法的不同点进行挂钩。而要不要挂钩，又由子类去决定。</p> <p>是不是很有用呢，我们再看下咖啡的制作</p> <div class="language-java extra-class"><pre class="language-java"><code><span class="token keyword">public</span> <span class="token keyword">class</span> <span class="token class-name">Coffee</span> <span class="token keyword">extends</span> <span class="token class-name">Drinks</span> <span class="token punctuation">{</span>
    <span class="token annotation punctuation">@Override</span>
    <span class="token keyword">void</span> <span class="token function">brew</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
        <span class="token class-name">System</span><span class="token punctuation">.</span>out<span class="token punctuation">.</span><span class="token function">println</span><span class="token punctuation">(</span><span class="token string">&quot;冲咖啡粉&quot;</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    <span class="token punctuation">}</span>

    <span class="token annotation punctuation">@Override</span>
    <span class="token keyword">void</span> <span class="token function">addCondiments</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
        <span class="token class-name">System</span><span class="token punctuation">.</span>out<span class="token punctuation">.</span><span class="token function">println</span><span class="token punctuation">(</span><span class="token string">&quot;加奶加糖&quot;</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
    <span class="token punctuation">}</span>
		<span class="token comment">//覆盖了钩子，提供了自己的询问功能，让用户输入是否需要加料</span>
    <span class="token keyword">boolean</span> <span class="token function">customerLike</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
        <span class="token class-name">String</span> answer <span class="token operator">=</span> <span class="token function">getUserInput</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
        <span class="token keyword">if</span> <span class="token punctuation">(</span>answer<span class="token punctuation">.</span><span class="token function">toLowerCase</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">.</span><span class="token function">startsWith</span><span class="token punctuation">(</span><span class="token string">&quot;y&quot;</span><span class="token punctuation">)</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
            <span class="token keyword">return</span> <span class="token boolean">true</span><span class="token punctuation">;</span>
        <span class="token punctuation">}</span> <span class="token keyword">else</span> <span class="token punctuation">{</span>
            <span class="token keyword">return</span> <span class="token boolean">false</span><span class="token punctuation">;</span>
        <span class="token punctuation">}</span>
    <span class="token punctuation">}</span>

    <span class="token comment">//处理用户的输入</span>
    <span class="token keyword">private</span> <span class="token class-name">String</span> <span class="token function">getUserInput</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
        <span class="token class-name">String</span> answer <span class="token operator">=</span> <span class="token keyword">null</span><span class="token punctuation">;</span>
        <span class="token class-name">System</span><span class="token punctuation">.</span>out<span class="token punctuation">.</span><span class="token function">println</span><span class="token punctuation">(</span><span class="token string">&quot;您想要加奶加糖吗？输入 YES 或 NO&quot;</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
        <span class="token class-name">BufferedReader</span> reader <span class="token operator">=</span> <span class="token keyword">new</span> <span class="token class-name">BufferedReader</span><span class="token punctuation">(</span><span class="token keyword">new</span> <span class="token class-name">InputStreamReader</span><span class="token punctuation">(</span><span class="token class-name">System</span><span class="token punctuation">.</span>in<span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
        <span class="token keyword">try</span> <span class="token punctuation">{</span>
            answer <span class="token operator">=</span> reader<span class="token punctuation">.</span><span class="token function">readLine</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
        <span class="token punctuation">}</span> <span class="token keyword">catch</span> <span class="token punctuation">(</span><span class="token class-name">IOException</span> e<span class="token punctuation">)</span> <span class="token punctuation">{</span>
            e<span class="token punctuation">.</span><span class="token function">printStackTrace</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
        <span class="token punctuation">}</span>
        <span class="token keyword">if</span> <span class="token punctuation">(</span>answer <span class="token operator">==</span> <span class="token keyword">null</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
            <span class="token keyword">return</span> <span class="token string">&quot;no&quot;</span><span class="token punctuation">;</span>
        <span class="token punctuation">}</span>
        <span class="token keyword">return</span> answer<span class="token punctuation">;</span>
    <span class="token punctuation">}</span>
<span class="token punctuation">}</span>
</code></pre></div><p>接着再去测试下代码，看看结果吧。</p> <p><img src="https://tva1.sinaimg.cn/large/007S8ZIlly1gih3og312yj315y07uta9.jpg" alt=""></p> <p>我想你应该知道钩子的好处了吧，它可以作为条件控制，影响抽象类中的算法流程，当然也可以什么都不做。</p> <p>模板方法有很多种实现，有时看起来可能不是我们所谓的“中规中矩”的设计。接下来我们看下 JDK 和 Spring 中是怎么使用模板方法的。</p> <h3 id="jdk-中的模板方法"><a href="#jdk-中的模板方法" class="header-anchor">#</a> JDK 中的模板方法</h3> <p>我们写代码经常会用到 <strong>comparable</strong> 比较器来对数组对象进行排序，我们都会实现它的 <code>compareTo()</code> 方法，之后就可以通过 <code>Collections.sort()</code> 或者 <code>Arrays.sort()</code> 方法进行排序了。</p> <p>具体的实现类就不写了(可以去 github：starfish-learning 上看我的代码)，看下使用</p> <div class="language-java extra-class"><pre class="language-java"><code><span class="token annotation punctuation">@Override</span>
<span class="token keyword">public</span> <span class="token keyword">int</span> <span class="token function">compareTo</span><span class="token punctuation">(</span><span class="token class-name">Object</span> o<span class="token punctuation">)</span> <span class="token punctuation">{</span>
    <span class="token class-name">Coffee</span> coffee <span class="token operator">=</span> <span class="token punctuation">(</span><span class="token class-name">Coffee</span><span class="token punctuation">)</span> o<span class="token punctuation">;</span>
    <span class="token keyword">if</span><span class="token punctuation">(</span><span class="token keyword">this</span><span class="token punctuation">.</span>price <span class="token operator">&lt;</span> <span class="token punctuation">(</span>coffee<span class="token punctuation">.</span>price<span class="token punctuation">)</span><span class="token punctuation">)</span><span class="token punctuation">{</span>
        <span class="token keyword">return</span> <span class="token operator">-</span><span class="token number">1</span><span class="token punctuation">;</span>
    <span class="token punctuation">}</span><span class="token keyword">else</span> <span class="token keyword">if</span><span class="token punctuation">(</span><span class="token keyword">this</span><span class="token punctuation">.</span>price <span class="token operator">==</span> coffee<span class="token punctuation">.</span>price<span class="token punctuation">)</span><span class="token punctuation">{</span>
        <span class="token keyword">return</span> <span class="token number">0</span><span class="token punctuation">;</span>
    <span class="token punctuation">}</span><span class="token keyword">else</span><span class="token punctuation">{</span>
        <span class="token keyword">return</span> <span class="token number">1</span><span class="token punctuation">;</span>
    <span class="token punctuation">}</span>
<span class="token punctuation">}</span>
</code></pre></div><div class="language-java extra-class"><pre class="language-java"><code><span class="token keyword">public</span> <span class="token keyword">static</span> <span class="token keyword">void</span> <span class="token function">main</span><span class="token punctuation">(</span><span class="token class-name">String</span><span class="token punctuation">[</span><span class="token punctuation">]</span> args<span class="token punctuation">)</span> <span class="token punctuation">{</span>
  <span class="token class-name">Coffee</span><span class="token punctuation">[</span><span class="token punctuation">]</span> coffees <span class="token operator">=</span> <span class="token punctuation">{</span><span class="token keyword">new</span> <span class="token class-name">Coffee</span><span class="token punctuation">(</span><span class="token string">&quot;星冰乐&quot;</span><span class="token punctuation">,</span><span class="token number">38</span><span class="token punctuation">)</span><span class="token punctuation">,</span>
                      <span class="token keyword">new</span> <span class="token class-name">Coffee</span><span class="token punctuation">(</span><span class="token string">&quot;拿铁&quot;</span><span class="token punctuation">,</span><span class="token number">32</span><span class="token punctuation">)</span><span class="token punctuation">,</span>
                      <span class="token keyword">new</span> <span class="token class-name">Coffee</span><span class="token punctuation">(</span><span class="token string">&quot;摩卡&quot;</span><span class="token punctuation">,</span><span class="token number">35</span><span class="token punctuation">)</span><span class="token punctuation">}</span><span class="token punctuation">;</span>
 
  <span class="token class-name">Arrays</span><span class="token punctuation">.</span><span class="token function">sort</span><span class="token punctuation">(</span>coffees<span class="token punctuation">)</span><span class="token punctuation">;</span>

  <span class="token keyword">for</span> <span class="token punctuation">(</span><span class="token class-name">Coffee</span> coffee1 <span class="token operator">:</span> coffees<span class="token punctuation">)</span> <span class="token punctuation">{</span>
    <span class="token class-name">System</span><span class="token punctuation">.</span>out<span class="token punctuation">.</span><span class="token function">println</span><span class="token punctuation">(</span>coffee1<span class="token punctuation">)</span><span class="token punctuation">;</span>
  <span class="token punctuation">}</span>

<span class="token punctuation">}</span>
</code></pre></div><p><img src="https://tva1.sinaimg.cn/large/007S8ZIlly1giha6rya5nj319006adhb.jpg" alt=""></p> <p>你可能会说，这个看着不像我们常规的模板方法，是的。我们看下比较器实现的步骤</p> <ol><li>构建对象数组</li> <li>通过 Arrays.sort 方法对数组排序，传参为 <code>Comparable</code> 接口的实例</li> <li>比较时候会调用我们的实现类的 <code>compareTo()</code> 方法</li> <li>将排好序的数组设置进原数组中，排序完成</li></ol> <p>一脸懵逼，这个实现竟然也是模板方法。</p> <p>这个模式的重点在于提供了一个固定算法框架，并让子类实现某些步骤，虽然使用继承是标准的实现方式，但通过回调来实现，也不能说这就不是模板方法。</p> <p>其实并发编程中最常见，也是面试必问的 AQS 就是一个典型的模板方法。</p> <h3 id="spring-中的模板方法"><a href="#spring-中的模板方法" class="header-anchor">#</a> Spring 中的模板方法</h3> <p>Spring 中的设计模式太多了，而且大部分扩展功能都可以看到模板方式模式的影子。</p> <p>我们看下 IOC 容器初始化时中的模板方法，不管是 XML 还是注解的方式，对于核心容器启动流程都是一致的。</p> <p><code>AbstractApplicationContext</code> 的 <code>refresh</code> 方法实现了 IOC 容器启动的主要逻辑。</p> <p>一个 <code>refresh()</code> 方法包含了好多其他步骤方法，像不像我们说的 <strong>模板方法</strong>，<code>getBeanFactory()</code> 、<code>refreshBeanFactory()</code> 是子类必须实现的抽象方法，<code>postProcessBeanFactory()</code> 是钩子方法。</p> <div class="language-java extra-class"><pre class="language-java"><code><span class="token keyword">public</span> <span class="token keyword">abstract</span> <span class="token keyword">class</span> <span class="token class-name">AbstractApplicationContext</span> <span class="token keyword">extends</span> <span class="token class-name">DefaultResourceLoader</span>
      <span class="token keyword">implements</span> <span class="token class-name">ConfigurableApplicationContext</span> <span class="token punctuation">{</span>
	<span class="token annotation punctuation">@Override</span>
	<span class="token keyword">public</span> <span class="token keyword">void</span> <span class="token function">refresh</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token keyword">throws</span> <span class="token class-name">BeansException</span><span class="token punctuation">,</span> <span class="token class-name">IllegalStateException</span> <span class="token punctuation">{</span>
		<span class="token keyword">synchronized</span> <span class="token punctuation">(</span><span class="token keyword">this</span><span class="token punctuation">.</span>startupShutdownMonitor<span class="token punctuation">)</span> <span class="token punctuation">{</span>
			<span class="token function">prepareRefresh</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
			<span class="token class-name">ConfigurableListableBeanFactory</span> beanFactory <span class="token operator">=</span> <span class="token function">obtainFreshBeanFactory</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
			<span class="token function">prepareBeanFactory</span><span class="token punctuation">(</span>beanFactory<span class="token punctuation">)</span><span class="token punctuation">;</span>
            <span class="token function">postProcessBeanFactory</span><span class="token punctuation">(</span>beanFactory<span class="token punctuation">)</span><span class="token punctuation">;</span>
            <span class="token function">invokeBeanFactoryPostProcessors</span><span class="token punctuation">(</span>beanFactory<span class="token punctuation">)</span><span class="token punctuation">;</span>
            <span class="token function">registerBeanPostProcessors</span><span class="token punctuation">(</span>beanFactory<span class="token punctuation">)</span><span class="token punctuation">;</span>
            <span class="token function">initMessageSource</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
            <span class="token function">initApplicationEventMulticaster</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
            <span class="token function">onRefresh</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
            <span class="token function">registerListeners</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
            <span class="token function">finishBeanFactoryInitialization</span><span class="token punctuation">(</span>beanFactory<span class="token punctuation">)</span><span class="token punctuation">;</span>
            <span class="token function">finishRefresh</span><span class="token punctuation">(</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
		<span class="token punctuation">}</span>
	<span class="token punctuation">}</span>
    <span class="token comment">// 两个抽象方法</span>
    <span class="token annotation punctuation">@Override</span>
	<span class="token keyword">public</span> <span class="token keyword">abstract</span> <span class="token class-name">ConfigurableListableBeanFactory</span> <span class="token function">getBeanFactory</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token keyword">throws</span> 		<span class="token class-name">IllegalStateException</span><span class="token punctuation">;</span>	
    
    <span class="token keyword">protected</span> <span class="token keyword">abstract</span> <span class="token keyword">void</span> <span class="token function">refreshBeanFactory</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token keyword">throws</span> <span class="token class-name">BeansException</span><span class="token punctuation">,</span> <span class="token class-name">IllegalStateException</span><span class="token punctuation">;</span>
    
    <span class="token comment">//钩子方法</span>
    <span class="token keyword">protected</span> <span class="token keyword">void</span> <span class="token function">postProcessBeanFactory</span><span class="token punctuation">(</span><span class="token class-name">ConfigurableListableBeanFactory</span> beanFactory<span class="token punctuation">)</span> <span class="token punctuation">{</span>
	<span class="token punctuation">}</span>
 <span class="token punctuation">}</span>
</code></pre></div><p>打开你的 IDEA，我们会发现常用的 <code>ClassPathXmlApplicationContext</code> 和 <code>AnnotationConfigApplicationContext</code> 启动入口，都是它的实现类（子类的子类的子类的...）。</p> <p><code>AbstractApplicationContext</code>的一个子类 <code>AbstractRefreshableWebApplicationContext</code> 中有钩子方法 <code>onRefresh()</code>的实现：</p> <div class="language-java extra-class"><pre class="language-java"><code><span class="token keyword">public</span> <span class="token keyword">abstract</span> <span class="token keyword">class</span> <span class="token class-name">AbstractRefreshableWebApplicationContext</span> <span class="token keyword">extends</span> …… <span class="token punctuation">{</span>
    <span class="token comment">/**
	 * Initialize the theme capability.
	 */</span>
	<span class="token annotation punctuation">@Override</span>
	<span class="token keyword">protected</span> <span class="token keyword">void</span> <span class="token function">onRefresh</span><span class="token punctuation">(</span><span class="token punctuation">)</span> <span class="token punctuation">{</span>
		<span class="token keyword">this</span><span class="token punctuation">.</span>themeSource <span class="token operator">=</span> <span class="token class-name">UiApplicationContextUtils</span><span class="token punctuation">.</span><span class="token function">initThemeSource</span><span class="token punctuation">(</span><span class="token keyword">this</span><span class="token punctuation">)</span><span class="token punctuation">;</span>
	<span class="token punctuation">}</span>
<span class="token punctuation">}</span>
</code></pre></div><p>看下大概的类图：</p> <p><img src="https://tva1.sinaimg.cn/large/007S8ZIlly1giie97gicdj31c20u0hdt.jpg" alt=""></p> <h2 id="小总结"><a href="#小总结" class="header-anchor">#</a> 小总结</h2> <p><strong>优点</strong>：1、封装不变部分，扩展可变部分。 2、提取公共代码，便于维护。 3、行为由父类控制，子类实现。</p> <p><strong>缺点</strong>：每一个不同的实现都需要一个子类来实现，导致类的个数增加，使得系统更加庞大。</p> <p><strong>使用场景</strong>： 1、有多个子类共有的方法，且逻辑相同。 2、重要的、复杂的方法，可以考虑作为模板方法。</p> <p><strong>注意事项</strong>：为防止恶意操作，一般模板方法都加上 final 关键词。</p> <h2 id="参考"><a href="#参考" class="header-anchor">#</a> 参考：</h2> <p>《Head First 设计模式》、《研磨设计模式》</p> <p>https://sourcemaking.com/design_patterns/template_method</p> <p><img src="https://cdn.jsdelivr.net/gh/Jstarfish/picBed/img/20200907141047.png" alt=""></p></div> <footer class="page-edit" style="display:none;"><!----> <!----></footer> <!----> <!----> <!----></main> <!----></div></div></div></div><div class="global-ui"><div class="back-to-ceiling" style="right:1rem;bottom:6rem;width:2.5rem;height:2.5rem;border-radius:.25rem;line-height:2.5rem;display:none;" data-v-db14854a data-v-db14854a><svg t="1574745035067" viewBox="0 0 1024 1024" version="1.1" xmlns="http://www.w3.org/2000/svg" p-id="5404" class="icon" data-v-db14854a><path d="M526.60727968 10.90185116a27.675 27.675 0 0 0-29.21455937 0c-131.36607665 82.28402758-218.69155461 228.01873535-218.69155402 394.07834331a462.20625001 462.20625001 0 0 0 5.36959153 69.94390903c1.00431239 6.55289093-0.34802892 13.13561351-3.76865779 18.80351572-32.63518765 54.11355614-51.75690182 118.55860487-51.7569018 187.94566865a371.06718723 371.06718723 0 0 0 11.50484808 91.98906777c6.53300375 25.50556257 41.68394495 28.14064038 52.69160883 4.22606766 17.37162448-37.73630017 42.14135425-72.50938081 72.80769204-103.21549295 2.18761121 3.04276886 4.15646224 6.24463696 6.40373557 9.22774369a1871.4375 1871.4375 0 0 0 140.04691725 5.34970492 1866.36093723 1866.36093723 0 0 0 140.04691723-5.34970492c2.24727335-2.98310674 4.21612437-6.18497483 6.3937923-9.2178004 30.66633723 30.70611158 55.4360664 65.4791928 72.80769147 103.21549355 11.00766384 23.91457269 46.15860503 21.27949489 52.69160879-4.22606768a371.15156223 371.15156223 0 0 0 11.514792-91.99901164c0-69.36717486-19.13165746-133.82216804-51.75690182-187.92578088-3.42062944-5.66790279-4.76302748-12.26056868-3.76865837-18.80351632a462.20625001 462.20625001 0 0 0 5.36959269-69.943909c-0.00994388-166.08943902-87.32547796-311.81420293-218.6915546-394.09823051zM605.93803103 357.87693858a93.93749974 93.93749974 0 1 1-187.89594924 6.1e-7 93.93749974 93.93749974 0 0 1 187.89594924-6.1e-7z" p-id="5405" data-v-db14854a></path><path d="M429.50777625 765.63860547C429.50777625 803.39355007 466.44236686 1000.39046097 512.00932183 1000.39046097c45.56695499 0 82.4922232-197.00623328 82.5015456-234.7518555 0-37.75494459-36.9345906-68.35043303-82.4922232-68.34111062-45.57627738-0.00932239-82.52019037 30.59548842-82.51086798 68.34111062z" p-id="5406" data-v-db14854a></path></svg></div><!----></div></div>
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